First, you will need the YouTube video embed link/URL.
1) On the YouTube video, click "Share".
2) Click "Embed".
3) Copy the link as shown between the quotes.
4) Start loading your advert as normal.
5) When you get to the "Media" tab, you'll now have the option to add/paste the link.
6) Proceed to finish loading your advert and your video will now be available.